A Randomized, Concurrent Controlled Trial to Assess the Safety and Effectiveness of the Separator 3D as a Component of the Penumbra System in the Revascularization of Large Vessel Occlusion in Acute Ischemic Stroke

Inclusion Criteria: * From 18 to 85 years of age * Present with symptoms consistent with an acute ischemic stroke for revascularization within 8 hours from symptom onset * Refractory to or not eligible for IV rtPA therapy., e.g., presenting between 0 and 3 hours from symptom onset AND contraindicated for IV rtPA, or presenting between 3 and 8 hours of symptom onset, or evidence of persistent occlusion from vascular imaging after IV rtPA * Evidence of a large vessel (2.5mm or greater in diameter) occlusion in the cerebral circulation * NIH Stroke Scale (NIHSS) score 8 or more points * Signed informed consent Exclusion Criteria: * History of stroke in the past 3 months. * Females who are pregnant * Pre-existing neurological or psychiatric disease that could confound the study results such as a pre-stroke mRS score 1 or higher * Known severe allergy to contrast media * Uncontrolled hypertension (defined as systolic blood pressure \>185 mmHg or diastolic blood pressure \>110 mmHg) * CT evidence of the following conditions at randomization: * Significant mass effect with midline shift * Large infarct region \>1/3 of the middle cerebral artery territory * Evidence of intracranial hemorrhage * Angiographic evidence of an arterial stenosis proximal to the occlusion that could prevent thrombus removal * Angiographic evidence of preexisting arterial injury * Rapidly improving neurological status prior to enrollment * Bilateral stroke * Intracranial tumors * Known history of cerebral aneurysm or arteriovenous malformation * Known hemorrhagic diathesis, coagulation deficiency, or on anticoagulant therapy with an International Normalized Ratio (INR) of \>1.7 * Baseline platelets \<50,000 * Use of IV heparin in the past 48 hours with PTT \>1.5 times the normalized ratio * Baseline glucose \<50mg/dL or \>300mg/dL * Life expectancy less than 90 days prior to stroke onset * Participation in another clinical investigation that could confound the evaluation of the study device